For the purpose of the present disclosure, the term “low voltage” refers to applications with operating voltages up to 1000V AC/1500V DC.
As known, switching devices used in low voltage circuits, such as circuit breakers, disconnectors, and contactors, are protection devices designed to allow the correct operation of specific parts of the electric circuits in which they are installed, and of electric loads connected to such electric circuits or parts thereof.
For instance, they ensure the availability of the nominal current necessary for several utilities, enable the proper insertion and disconnection of loads from the circuit, and protect (especially for circuit breakers) the grid and the loads installed therein against fault events such as overloads and short circuits.
Numerous industrial solutions for the aforementioned devices are available on the market.
Known electro-mechanical switching devices generally have a case housing one or more electric poles, which each include a couple of separable contacts to make, break and conduct current. A driving mechanism causes the movable contacts to move between a first closed position in which they are coupled to the corresponding fixed contacts and a second open position in which they are spaced away from the corresponding fixed contacts.
In the closed position, well-designed contacts result in quite low power losses, whereas in the open position, they guarantee galvanic (electrical) isolation of the downstream circuit provided that the physical separation between the contacts are above a minimum value. Such galvanic isolation is important in common electrical practice, because it enables safe repairing and maintenance works on the circuit in which the switching device is inserted.
Although such known switching devices have proven to be very robust and reliable, in direct current (“DC”) applications, and mainly at relatively high voltage (up to 1500V), the interruption time can be quite high, and therefore electric arcs which usually strike between mechanical contacts under separation may consequently last long.
Such long arcing times result in severe wear of the contacts, thus reducing significantly the electrical endurance, such as the number of switching operations that a switching device can perform.
In order to address such issues in DC applications, there have been designed so-called Solid-State Circuit Breakers (SSCBs) which use Power Electronics Switches (PES), using semiconductor-based power devices, such as Power MOSFETs, Insulated Gate Bipolar Transistors (IGBTs), Gate Turn-Off Thyristors (GTO) or Integrated Gate-Commutated Thyristors (IGCTs), that can be turned on and off by means of an electronics driving unit so as to have arcless current making and breaking operations.
The main advantage of such SSCBs is that they have potentially unlimited electrical endurance due to arcless operations. On the other hand, PES devices suitable for high currents, for example, above 100 A, have very high on-state conduction losses.
Therefore, SSCBs waste quite a lot of energy and require intensive cooling to remove the heat generated and keep the temperature at safe levels.
In order to mitigate these problems, there have been devised hybrid solutions where a known or main switching (“MS”) device is connected in parallel to a PES device. The main switching device conducts the current in normal operations, while the PES device is only used at breaking or making current.
Such hybrid solutions have low power losses, in principle not higher than those of known switching devices, and therefore do not require special cooling also when continuously loaded at full power.
However, a drawback of PES devices is that in an off-state, if a voltage is applied to their terminals, for example, an anode and cathode for an IGCT, or collector and emitter for an IGBT, they conduct a small current (leakage current), for example, up to a few dozens of mA. As a result, SSCBs and hybrid solutions also have limited power losses in an open state and are not suitable for galvanic isolation.
This severe limitation can be avoided by means of another known switch referred to as an Isolation switch (IS) which is serially connected to the PES device.
The proper working of such complex device requires that the IS, MS and PES devices are operated in a very strict sequence and with tight timing in breaking and making operations. For example, in normal operating conditions, the MS and IS devices are closed, and the PES is in an off-state. When it is necessary to interrupt the flow of current (opening or current breaking operation), the PES device turns-on (with no current passing through, because the voltage across the device, for example, the voltage drop on the MS device, is generally lower than a threshold voltage, which is the Collector-Emitter Voltage (VCE) in the case of IGBTs and the On-State Voltage (VT) in the case of IGCTs), the MS device opens and an arc is ignited between its contacts. The arc voltage diverts the current towards the PES device, and the arc between the contacts of the MS device is extinguished right after. The PES device turns off breaking the main current, wherein this step should be executed only when the distance between the contacts of the MS device is large enough to avoid an arc reignition. Just after the IS device opens which also breaks the leakage current. When instead, it is necessary to close the contacts (current making operations), starting from a condition where the MS and IS devices are open, and the PES device is in an off-state, the IS device is closed first, thus making only the low leakage current. Then, the PES device turns-on making the main or nominal current, and after the MS device closes thus diverting the current from the PES device with a small arc between the contacts of the MS device itself.
In practice, the isolation switch device makes or breaks only small leakage currents, for example, smaller than 100 mA, and wear of its contacts is negligible. In the same way, the contacts of the MS device are also exposed only to small and short arcs and their wear is significantly reduced in comparison with traditional mechanical switchgear.
As a result, with hybrid solutions, a much higher number of electrical make or break operations even with high currents can be executed.
Notwithstanding, there is still a desire for further improvements of known hybrid solutions, such as with regard to simplifying their constructive layout, realizing a better synchronized coordination of their operations, and maintaining such synchronization over a longer and possible the entire working life.